Simplify a complicated mesh for export as an STL
I agree with Jachym.   The remesh modifier may do the trick.   

In object-mode, select Modifiers, and add the Remesh modifier.  Find the Octree Depth Option (it will be the main one for your issue) and adjust it up or down.  Careful! With high poly-counts like you have - adjusting it higher may cause Blender to crash so take the necessary precautions with this in mind.

See this vid at the 1:30 timeframe to see it in action.

https://video.search.yahoo.com/yhs/search?fr=yhs-symantec-ext_onb&amp;hsimp=yhs-ext_onb&amp;hspart=symantec&amp;p=tutor4u+blender+text#id=5&amp;vid=79e85e222216fa41a1ec00cee407affe&amp;action=view